Issue with normal mapping - swift

Question

I'm working on implementing normal mapping (using Swift/Metal). However, I think I've made a mistake when calculating the tangent and bitangent, and I can't find the issue.
Here is a screenshot with normal mapping:
basic shading with normal map & basic shading, normal mapping normals
And here is a screenshot only using the normals provided by the obj file:
basic shading without normal map, normals.
Here is the code for calculating the tangent and bitangent:
func calculateTangentAndBitangent(v1: VertexData, v2: VertexData, v3: VertexData) -> (vec3, vec3) {
let pos1 = v1.position
let pos2 = v2.position
let pos3 = v3.position
let uv1 = v1.uv
let uv2 = v2.uv
let uv3 = v3.uv
let e1 = pos2 - pos1
let e2 = pos3 - pos1
let x1 = uv2.x - uv1.x
let x2 = uv3.x - uv1.x
let y1 = uv2.y - uv1.y
let y2 = uv3.y - uv1.y
let r = 1.0 / (x1 * y2 - x2 * y1)
let tangent = (e1 * y2 - e2 * y1) * r
let bitangent = (e2 * x1 - e1 * x2) * r
return (tangent, bitangent)
}
for i in stride(from: 0, to: vertexBuffer.count, by: 3) {
let v1 = vertexBuffer[i]
let v2 = vertexBuffer[i+1]
let v3 = vertexBuffer[i+2]
let tb = calculateTangentAndBitangent(v1: v1, v2: v2, v3: v3)
tangents[normalIndices[i]] += tb.0
tangents[normalIndices[i+1]] += tb.0
tangents[normalIndices[i+2]] += tb.0
bitangents[normalIndices[i]] += tb.1
bitangents[normalIndices[i+1]] += tb.1
bitangents[normalIndices[i+2]] += tb.1
}
for i in 0..<vertexBuffer.count {
var v = vertexBuffer[i]
v.tangent = normalize(tangents[normalIndices[i]])
v.bitangent = normalize(bitangents[normalIndices[i]])
vertices.append(v.vertex)
}
Vertex shader:
out.tangent = normalize((modelUniforms.modelMatrix * float4(in.tangent, 0)).xyz);
out.bitangent = normalize((modelUniforms.modelMatrix * float4(in.bitangent, 0)).xyz);
out.normal = normalize((modelUniforms.modelMatrix * float4(in.normal, 0)).xyz);
Fragment shader:
constexpr sampler linearSampler(mip_filter::linear,
mag_filter::linear,
min_filter::linear);
float3 normalSample = float3(normalMap.sample(linearSampler, in.texCoord).xyz);
normalSample = normalize(normalSample * 2 - 1);
float3 normal = normalize(in.tangent * normalSample.x +
in.bitangent * normalSample.y +
in.normal * normalSample.z);
float3 L = normalize(sceneUniforms.sun.direction);
float3 NdL = max(dot(in.normal, L), 0.0);
half3 diff = half3(NdL);
I've spent a lot of time looking into this and I still couldn't figure out what the issue is.

Answer 1

If you use MTKTextureLoader to load your textures make sure you specify MTKTextureLoader.Option.SRGB: false when loading normal maps so that the image data is treated as linear pixel data, else the normals will be all wrong.
https://developer.apple.com/documentation/metalkit/mtktextureloader/option/1536032-srgb